Year	micrograms per cubic meter
1990	40.0462231
1995	38.01561498
2000	35.86654107
2005	29.198214
2010	28.87738634
2011	32.544288
2012	30.32467663
2013	32.24187476
2014	29.42704565
2015	30.94349566
2016	29.17182245
2017	27.81052009
2018	28.02331277
2019	27.64788197